While carrying out servicing or maintenance operations on a workpiece or piece of equipment, it is often necessary or desirable to be able to rotate the workpiece to a variety of positions. Sometimes this is necessary to make the work area accessible to the operator or workman. In various kinds of welding in particular it is frequently desirable that the part being worked on be freely movable, often to permit the welder to control the gravity flow of welded metal and fused metal about a weld.
Support structures which permit a workpiece to be rotated are well known and are commonly referred to as roll-over fixtures. For example, one known fixture or workholder has a primary supporting structure and workholding means mounted for rotation in the supporting structure about a fixed axis of rotation. The workholding means consists of a pair of spaced-apart workholding elements providing a work-receiving area therebetween. An apparatus is generally provided for rotating the workholding means and thus the workpiece as desired. When the workpiece is initially mounted in the workholder, generally in an upright position, the workpiece can be arranged so that its centre of gravity is located along the plane formed by the centre lines of the two workholding elements, these centre lines extending in the vertical direction through the centre of the elements when they are in an upright position. Thus there is little or no turning torque due to the weight of the owrkpiece on the workholding elements initially.
With known workholders however, as soon as the workpiece is rotated from its initial position, a turning torque may be set up by the weight of the workpiece due to the centre of gravity of the workpiece not being located on the axis of rotation thereof. This turning torque will either make it more difficult to rotate the workpiece if the torque is in the opposite direction as the direction of rotation or it will make it easier to rotate the workpiece if the torque is in the same direction as the direction of rotation. Therefore the turning torque of the centre of gravity may require that a power source capable of providing a relatively large amount of power be used to turn the workpiece than would otherwise be the case if the centre of gravity was located on the axis of rotation. Alternatively the turning torque of the centre of gravity could give rise to a runaway condition that might damage either the workpiece or workholder or both. For example, if an engine is used to rotate the workpiece, this engine might be caused to overspeed because of the turning torque, damaging the engine possibly. The turning torque of the centre of gravity can also set up high stress levels in either the components of the workpiece or workholder as the workpiece is rotated.
The problems arising from the turning torque of the centre of gravity are generally not as severe with small workpieces obviously. With such workpieces it is sometimes possible to arrange the centre of gravity on the axis of rotation of the workholder effectively eliminating any undesirable turning torque. It is also easier to build the components of the workholder so that they can withstand any possible turning torque from the relatively small weight of the workpiece.
However severe "turning torque" problems can arise where the workpiece to be serviced or worked on is quite large in size and heavy. Examples of such workpieces would include massive electric wheel motors for large off-highway trucks, large transmissions and generators for such vehicles and locomotive engines. Large workholders or roll-over fixtures are often required to service and maintain such equipment. The weight of such equipment is in the order of five tons or more. It may be difficult to locate the centre of gravity of such machinery on the axis of rotation of the roll-over fixture and, when it is not so located, great turning torques can be set up in the fixture as soon as the workpiece in question is rotated to any significant extent. Clearly too, a motor of some sort is employed to rotate such workpieces.
It is an object of the present invention to provide a workholder which substantially overcomes the abovementioned problem arising from the inherent turning torque of the weight of the workpiece placed in the workholder.
It is a further object of the invention to provide a workholder which overcomes this problem by means including a control system comprising sensing means sensitive to out of balance forces within the workholder caused by rotation of a workpiece and providing an output signal indicative of these forces, and adjusting means responsive to this signal to effect positional adjustment of the workholding means.